Plasma-sprayed metallic/ceramic thermal barrier coatings utilizing stabilized zirconium oxide are widely used to protect metal components exposed to high temperature conditions and, in general, reduce both the temperature of the base metal and the effects of thermal transients. Such systems are commonly used in combustion chambers, transition ducts and after burner liners in gas turbine engines and may also be used in protecting the vane platforms and air foils in various stages.
The most important feature of these coatings is their thermal insulating properties, since the magnitude of reduction in base metal temperature and transient thermal stress is related to the low thermal conductivity of the oxide component and the thickness of the coatings. In general, the desired properties of a practical thermal barrier coating are as follows:
(a) low thermal conductivity;
(b) adequate, adherence for resistance to thermal stress spalling, i.e., good interparticle and substrate bonding is required;
(c) maximum metallurgic integrity and oxidative/hot corrosion resistance of the metallic constituent;
(d) closest possible thermal expansion match between the ceramic and the substrate alloy;
(e) adequate stabilization of the desired (cubic zirconia) crystal structure to minimize effects of the non-linear thermal expansion caused by structural transformation; and,
(f) repairability during manufacturing and after field service.
The current state-of-the-art employs several ceramic-metal systems based on magnesia stabilized zirconia. In general, the base metal is a nickel or cobalt-base superalloy such as Hastelloy X, TD-nickel, or Haynes 188 which is coated with a bond layer of nickel-5% Al or nickel-20% chromium alloy, an intermediate metallic, stabilized zirconia ceramic layer and a top layer of stabilized zirconia. These layers are plasma-sprayed onto the base and the art now recognizes that improved performance and lower application costs can be achieved with nominally continuous grading processing methods by which the concentration of the zirconia is continuously increased from 0, at the interface between the bond layer and the base metal, to substantially 100 percent at the outer surface. Generally, these coatings are applied to a thickness of about 15 mils.
Detailed discussions representative of these various techniques can be found in U.S. Pat. Nos. 3,006,782 dated Oct. 31, 1961, to Wheildon for Oxide Coated Articles With Metal Undercoatings; 2,937,102 dated May 17, 1960, to Wagner for Zirconia Stabilization Control; 3,091,548 dated May 28, 1963, to Dillon for High Temperature Coatings; and 3,522,064 dated July 28, 1970, to Valdsaar for Stabilized Zirconia Containing Niobia and Calcium Oxide.
At present, one of the favored ceramic components is zirconia which can be used either alone or admixed with a material such as magnesium oxide, calcium oxide, yttrium oxide, La.sub.2 O.sub.3, Ce.sub.2 O.sub.3, which are known to stabilize the zirconia in the more desirable cubic form. Accordingly, one of the best means for protecting nickel and cobalt-base superalloys from high temperature environments now known to the art, consists of a zirconia-based ceramic coating which is bonded to the base coating by a nickel-chromium or nickel-aluminum alloy in which the concentration of the ceramic increases either gradually or in discreet increments from the substrate to the outer coating.
While these advanced systems have been found to give good service, failures, when they did occur, were observed to be caused by oxidative degradation of the metallic constituent followed by exfoliation of the outer ceramic layers. Further, when failures did occur, repair of the items has been difficult because of the resistance of the metallic constituent to available acid-stripping solutions. According to this invention, we have found that proper selection of the bond coat metal produces substantial improvements in the performance of the thermal barrier as well as in the ease of repairability of the article.
It is, accordingly, an object of this invention to provide an improved ceramic/metallic thermal barrier coating for nickel and cobalt-base superalloys. This, and other objects of this invention, will be readily apparent from the following description: